ABSTRACT ? PROJECT 1 FTLD-tau exhibits imbalanced tau proteostasis (intra- and inter-cellar) and neuronal dysfunction. However, little is known about how the accumulation and spread of pathogenic tau could occur, and how it could affect neuronal and synaptic functions. As an integral part of the Center, project 1 (P1)'s objective is to dissect mechanisms underlying aberrant neuronal activity and tau distribution in FTD. We combine unbiased and hypothesis-driven approaches, which are extremely powerful but cannot be achieved without the structure of the Center. We propose three Specific Aims. In Aim 1, we will define FTD mutations-induced aberrant tau post-translational modifications (PTMs) and dissect how acetylation stimulates tau release with P3. Aim 1 will also be complemented by transcriptome analyses of the FTD mutation effects in P2. In Aim 2, we will determine how FTLD-tau alters neuronal activity and activity-dependent tau release. We will expand on our preliminary study that V337M human neurons exhibit hyperexcitability with impaired homeostatic regulation of axon initial segment (AIS). We will systematically compare neuronal and synaptic activity in isogenic FTD mutant and control lines by whole-cell recordings. Working with mass-spec (MS) core, we will also dissect the mechanism underlying the activity-dependent tau release by dissecting tau interactome in response to high KCL (MS core). CRISPRi/a will then be performed to examine how modulating hits from interactome study would affect activity-dependent tau release (CRISPR core). Aim 2 in P1 will be complemented by P2, which correlates neuronal activity with transcriptome, and P3, which examines the crosstalk between neuronal activity and tau degradation pathways. In Aim 3, we will determine how FTD mutations affect pathogenic tau seeding. Pathogenic seeding is a key feature in tauopathy, but little is known about its mechanisms and functional outcome. We will compare neuronal and synaptic activity in receiving human neurons positive or negative for FRET signal resulting from tau oligomerization. To dissect the mechanisms mediating pathogenic seeding, we will compare the proteome associated with internalized mutant or WT tau seeds in human neurons (MS core). Proteins selectively linked with mutant tau will be prioritized as potential hits. Working with CRISPR core, we will inhibit/activate selected genes from the seeding proteome, and measure the outcome by FRET signal and whole-cell recordings. The proteomic approach in Aim 3 will be complemented by the genome-wide screening to identify pathways in internalization using unbiased approach (P2 and CRISPR core). Together with P2 and P3, P1 will contribute to our overall vision of dissecting the crosstalk between tau proteostasis imbalance and neuronal dysfunction.